The present invention relates to coupling devices of the type used to transmit torque, for example in a vehicle drive line, and more particularly, to a method of controlling the transmission of torque in and through such a coupling device.
As used herein, the term “coupling device” will be understood to mean and include a device which is able to transmit torque from an input to one or more outputs, and in which there is a clutch assembly disposed between the input and the output, such that the amount of torque transmitted is a function of the extent of engagement of the clutch assembly. Within the scope of the present invention, the term “coupling device” means and includes both gear-type devices (such as differentials), as well as gearless-type couplings.
Although the control method of the present invention may be utilized with many different types and configurations of coupling devices such as a coupling made in accordance with the teachings of U.S. Pat. No. 5,964,126 assigned to the assignee of the present invention and incorporated herein by reference, it is especially advantageous when utilized in conjunction with vehicle differentials of the type illustrated and described in U.S. Pat. Nos. 5,310,388 and 6,464,056, both of which are also assigned to the assignee of the present invention and incorporated herein by reference.
In the differential coupling devices of the cited patents, there is a clutch pack operable to transmit torque between the input (housing connected to the ring gear) and the output (one of the axle shafts), with the degree of engagement of the clutch pack being determined by the fluid pressure in a piston chamber. The fluid pressure biases a clutch piston against the clutch pack. The differential coupling devices of the cited patents include a gerotor pump having one rotor fixed to rotate with the input and the other rotor fixed to rotate with the output, such that the flow of pressurized fluid into the clutch piston chamber is generally proportional to the speed difference between the input and the output. As used herein, the term “clutch pack” will be understood to mean and include both a multiple friction disc type clutch pack, as well as any of the other well known types of clutch assemblies, such as cone clutches, in which the degree of engagement is generally proportional to the fluid pressure acting on the clutch piston, or on an equivalent clutch-engagement structure.
In differential coupling devices of the type described above, it is typical to provide a flow path from the clutch piston chamber to a reservoir or some other source of low pressure fluid, and to provide in this flow path some sort of control valve which can control the flow from the clutch piston chamber to the low pressure source, thereby controlling the pressure in the clutch piston chamber, and therefore, controlling the “bias torque”, i.e., the extent to which torque is transmitted from the input to the output.
It has been known in prior art coupling devices of the type described above to provide a control valve of the type which comprises a flow metering type of device, in other words, a device which is essentially a variable flow restriction type of device. In such a device, fluid flows through a small orifice, with the pressure in the clutch piston chamber being determined by the “back pressure” created by the flow of fluid through the orifice. As has been recognized by those skilled in the art for some time now, one of the problems associated with this type of clutch pressure control is that the flow (and therefore, also the “clutch pressure”) is very much dependent upon factors such as fluid viscosity and fluid temperature which, in many devices of the type to which the present invention relates, will vary significantly during the period of operation of the device. Therefore, consistency of operation is difficult to achieve in coupling devices having the prior art type of pressure control system described above.
Increasingly, vehicle differential coupling devices of the type to which the present invention relates are being utilized in conjunction with ABS (“Anti-skid Braking Systems”), and various other types of active ride control and vehicle dynamics control systems. When a vehicle differential coupling device is utilized in conjunction with such systems, it is critical to be able to vary the bias torque (i.e., the degree of torque transmission) at a rate of response which is not only very fast, but which is also predictable and repeatable.